1. Field of the Invention
The present invention relates to an image forming method and image forming apparatus which use a developer containing a toner for developing an electrostatic image.
2. Description of the Related Art
Recently, downsizing of toner particles in diameter has been promoted to give images of higher quality, which is accompanied by problems resulting from decreased toner flowability. Toner particles become easy to flocculate as their flowability decreases, degrading their transferability and causing falling-off of the images obtained. The problems resulting from degraded transferability caused by decreased toner flowability are more pronounced in toners which contain a releasing agent, e.g., wax, to prevent twining of toner particles on a transfer medium around a fixing roller, or off-setting of toner. The twining means a phenomenon in which while toner images transferred onto a recording medium, e.g., transfer paper (hereinafter sometimes referred to as transfer medium) are fixed by a fixing roller or the like, a toner twines around the fixing roller, because it cannot be completely removed from the roller, or a recording medium cannot be smoothly discharged because it is caught by a separating nail, if provided. The off-setting means a phenomenon in which, while toner images transferred onto a recording medium are fixed by a fixing roller, the toner moves towards the fixing roller and contaminates the recording medium. Moreover, problems caused by degraded toner transferability are particularly pronounced in full-color toners used in full-color image forming apparatuses.
One-component developing devices, which are advantageous for reducing printer size, are strongly demanded to be smaller, from the view point of space-saving and increased freedom of printer setting. Each component in developing units is also required to be smaller, as toner transfer mechanisms in the devices become smaller. However, reduction in size of each component in the devices causes following problems.
Reducing diameters of developing and toner supply rollers, the former for developing toner images on an image bearing member and the latter for supplying a toner to a developing roller, will decrease toner supply rate, with the result that the device may not well keep up with toner requirements (i.e., insufficient toner followability) and produce uneven images. This problem resulting from the poor followability due to the decreased developing roller diameter is more pronounced when toner particles become smaller and still more pronounced with toners incorporated with a releasing agent, e.g., wax.
Use of a cleaner-less photoconductor is one of the measures for downsizing image forming apparatuses. Such a photoconductor can contribute to space-saving, because it needs no cleaning blade mechanism. However, use of a cleaner-less photoconductor causes problems in a transfer step carried out in multi-color image forming apparatuses, in which toner images developed on a latent image bearing member, e.g., photoconductor drum, are transferred sequentially onto a transfer medium, e.g., transfer paper, or intermediate transfer member, e.g., intermediate transfer belt, with which the latent image bearing member comes into contact. These problems are caused by reverse transfer, in which a toner image is transferred from the transfer medium back to the latent image bearing member while the subsequent image is transferred, to cause color contamination and degrade quality of multi-color images. The problems resulting from the reverse transfer are more pronounced in toners of reduced particle diameter.
Several attempts have been proposed to solve these problems. For example, Japanese Patent Application Laid-Open (JP-A) No. 2003-98846 discloses a proposal of a technique which uses a toner in which the surface of the resin base particle is coated with an external additive to have enhanced adherence between an image bearing member or intermediate transfer member and toner over that among toner particles for image forming apparatuses having an intermediate transfer member, to prevent reverse transfer caused by discharge at the transfer section and thereby to prevent serious image defects, e.g., image falling off. JP-A No. 2002-323786 proposes use of toners having non-electrostatic adherence of 15 nN or less and flocculation degree of 6% or more, to prevent reverse transfer of toner from a transfer medium, e.g., intermediate transfer belt, to a latent image bearing member, e.g., photoconductor drum.
However, the invention disclosed by JP-A No. 2003-98846, which merely specifies a relative adherence relationship, is difficult to sufficiently prevent reverse transfer. Meanwhile, the invention disclosed by JP-A No. 2002-323786 is also difficult to sufficiently prevent reverse transfer, because of insufficient adherence. In particular, these inventions disclosed by JP-A Nos. 2003-98846 and 2002-323786 are difficult to sufficiently prevent reverse transfer, when a toner composed of relatively small particles is used for one-component development in a full-color image forming apparatus, to cause color contamination and deterioration of multi-color image quality.